fbsd-nat: Correct the return type of the have_regset method.

During the development of 40c23d88,
the return value of fbsd_nat_target::have_regset was changed from a
simple boolean to returning the size of the register set.  The
comments and callers were all updated for this change, but the actual
return type was accidentally left as a bool.  This change fixes the
return type to be a size_t.

Current callers of this only checked the value against 0 and thus
still worked correctly.